Traditional spread spectrum clock generators have recently been developed to replace fixed clock generators, such as crystal oscillators, for reducing electromagnetic interference (“EMI”) in high-speed digital electronic systems. Spread spectrum clock generators suppress undesired tones by spreading signal energy of digitized data signals over bands of frequencies to reduce spectral power. Undesired harmonics usually give rise to unacceptable levels of EMI in high-speed digital applications, such as computer peripheral devices, consumer electronics, and embedded controller systems.
FIG. 1 depicts a typical system 100 implementing a conventional spread spectrum clock generator 122 for reducing EMI. System 100 includes a general-purpose graphics processing unit (“GPU”) 120 to generate digitized pixel data for rendering computer-generated graphical images. A liquid crystal display driver 102 then processes the pixel data to display the images via connection 103 on a liquid crystal display (“LCD”) 101. Fixed clock generator 124 controls the timing of the internal processes of GPU 120 and spread spectrum clock generator 122 generates a spread clock 106 for transmitting the pixel data over connection 104 at varying clock frequencies or data rates to minimize EMI.
While functional, the above-described spread spectrum clock generator and its approach to minimize EMI have several drawbacks that limit their use primarily to LCDs. One drawback is that conventional spread spectrum clock generators facilitate only one-way data transfers over connection 104. In particular, these generators are not configured to coordinate two-way data transfers. Another drawback is that conventional spread spectrum clock generators generally do not control the variability of the average data rate at which data arrives at the receiving end (e.g., at LCD display 101) of the spread spectrum clock signal. Consequently, spread clock 106 typically transmits data via connection 104 at average data rates that can drift faster or slower than a center frequency about which frequencies are spread. Also, spread clock 106 is usually the clock source for retransmitting data via connection 103. Note that the variation in the average data rates of the retransmitted data therefore varies the rates at which pixels are written to the display. Fortunately, the variability in the retransmission data rate to LCD display 101 is tolerable as the human eye cannot visually detect the effects of the different rates at which pixels are written the display. But to ensure the drift in average data rates over periods of time remain imperceptible to prevent in blurring of images, the amount of frequencies in the range of frequencies is limited to 2% to 3%, for example, about the center frequency. A drawback to limiting the number of frequencies results in less energy being spread over the range of frequencies, thereby limiting the amount of EMI that can be reduced. Another drawback to the above-described spread spectrum clock generator is that it is implemented to facilitate only unidirectional data transfers (e.g., from LCD driver 102 to display 101). It is not suited for establishing reliable bidirectional data transfers that are critical for a number of RF communication systems.
In view of the foregoing, it would be desirable to provide a variable-frequency clock generator that minimizes the above-mentioned drawbacks when implementing variable data rates to spread signal energy of data signals in high-speed digital links used in RF communications systems.